This invention relates to a refrigeration method and apparatus and is particularly concerned with the liquefaction of permanent gases such as nitrogen and methane.
Nitrogen and methane are permanent gases which cannot be liquefied solely by decreasing the temperature of the gas. It is necessary to cool it (at pressure) at least to a "critical temperature", at which the gas can exist in equilibrium with its liquid state.
Conventional processes for liquefying nitrogen or for cooling it to below the critical point typically require the gas to be compressed (unless it is already available at a suitably elevated pressure, generally a pressure above 30 atmospheres) and heat exchanged in one or more heat exchangers against at least one relatively low pressure stream of working fluid. At least some of the working fluid is provided at a temperature below the critical temperature of nitrogen. At least part of the stream or of each stream of working fluid is typically formed by compressing the working fluid, cooling it in the aforesaid heat exchanger or heat exchangers, and then expanding it with the performance of external work ("work expansion"). The working fluid is preferably taken from the high pressure stream of nitrogen, or this stream may be kept separate from the working fluid, which may nevertheless consist of nitrogen.
In practice, liquid nitrogen is stored or used at a pressure substantially lower than that at which the gaseous nitrogen is taken from isobaric cooling to below its critical temperature. Accordingly, after completing such isobaric cooling, the nitrogen at below its critical temperature is passed through an expansion or throttling valve whereby the pressure to which it is subjected is substantially reduced, and liquid nitrogen is thus produced together with a substantial volume of so called "flash gas". The expansion is substantially isenthalpic and results in a reduction in the temperature of the nitrogen being effected.
Generally, the thermodynamic efficiency of a conventional commercial process for liquefying nitrogen is relatively low and there is ample scope for improving such efficiency. Considerably emphasis in the art has been placed on improving the total efficiency of the process by improving the efficiency of heat exchange. Much analysis has been done of the temperature differences between the respective streams at various points in the heat exchangers to determine the overall thermodynamic efficiency of the heat exchange.
Our approach not only involves improving the efficiency of heat exchange but extends to providing a drastic reduction in the total heat duty of the exchangers, and extends further to improving the performance of the working fluid cycles as well. It is known in nitrogen liquefiers to employ two or more such working fluid cycles providing refrigeration over temperature ranges which are mutually adjacent but do not overlap, the so-called "series" configuration. See, for example, our U.S. Pat. Nos. 4,638,639 and 4,638,638. Thus in a series configuration a "warm turbine working fluid cycle" might involve refrigerating the product stream from 200K to 160K, an "intermediate turbine working fluid cycle" might refrigerate the product stream from 160K to 130K, and a "cold turbine working fluid cycle" might continue the cooling from 130K to 100K.
It is also possible to use just two turbines in a series arrangement, one turbine being part of a `warm turbine working fluid cycle` the other turbine being part of a `cold turbine working fluid cycle`. The adjectives `cold`, `intermediate` and `warm` as applied herein to turbines refer to the relative inlet temperatures of the turbines.